Apparatus of the type with which the invention is concerned is described in U.S. Pat. application Ser. No. 07/991,106 filed Dec. 16, 1992, now U.S. Pat. No. 5,271,297 in the names Siegfried Heiermann and Horst Werner Oehler. As described in the aforementioned U.S. Patent application when the screw-threaded elements or studs are to be released or if nuts are to be screwed onto or from the elements it is necessary to tension or brace the elements. To achieve this a carrier ring is set onto the cover of the vessel and piston and cylinder units are extended to urge bridge members against shoulders on screw-threaded sleeves fitted onto the screw-threaded elements. With the elements braced in this fashion the nuts can be slackened or tightened to achieve as uniform as possible clamping pressure on the cover. When the gaps between the screw-threaded elements disposed on a common pitch circle are relatively small the piston and cylinder units are usually disposed in pairs in a parallel manner outside the pitch circle with the screw-threaded elements therebetween. (See also DE-2428247 and U.S. Pat. No. 4216876). One problem encountered with the known clamping devices is to achieve uniform forces from the two units straddling each screw-threaded element axially to the latter without imposing torque. In practice this is difficult because the screw-threaded elements are often distorted by the clamping forces on the cover and these elements tend mainly to flex towards the outside rim of the cover. Also the flange of the vessel in which the threaded bores are provided can become deformed. It is also the case that the threaded bores are not always exactly positioned on the same common pitch circle. As a result of these various factors the two units of one mechanism are often not at exactly the same radial spacing from the screw-threaded element relative to the pitch circle and there is imbalance which gives rise to an undesired torque on the screw-threaded element.
An object of the present invention is to provide an improved form of clamping device or set of mechanisms.